The invention relates generally to the imaging of body passage surfaces through body fluids and, in particular, to a novel technique for imaging cardiovascular surfaces through blood. The invention also relates to an apparatus for carrying out a method of such kind. Such method and apparatus can be advantageously used in examination of cardiovascular surfaces and conducting minimally invasive procedures in cardiovascular surgery.
Direct visualization of body passages has become a routine procedure now. Modern endoscopic technique is used for viewing and imaging different body passages, such as gastrointestinal tract, bronchial passages, joints, cardiovascular system. Most of the passages are filled with body fluids, such as blood, urine, stomathic fluids which are opaque for illumination and prevent visualization of the passage""s surfaces. For passages not filled with blood it is not a problem because in such body cavities as stomach or esophagus the fluid can be evacuated to clear the visualization field. Similar technique is used in angioscopy. Sterile saline flush solution is introduced into the vessel continuously or periodically for blood evacuation (U.S. Pat. No. 4,998,972 issued Mar. 3, 1991 to Chin Albert K. et. al,; U.S. Pat. No. 4,175,545 issued November 1979 to Termanini). Such procedure provides user with very short time of vision determined by the length of irrigation period. It is particularly difficult to perform this procedure in arterial system where pressure and flow rate of blood are much higher than in veins. It makes difficult to obtain enough clear bloodless field for visualization of the cardiovascular surfaces.
Ability to see through blood would enable revolutionary new approaches in diagnosis and treatment of cardiac, arterial, and venous diseases.
In standard angioscopy procedure a light delivered into a lumen of a cardiovascular passage irradiates the interior. Only backscattered and reflected light is available for imaging. Radiation being reflected and scattered by the surface structure at least partly is detected and intensity signals are used to produce image signals. High concentration of blood cells along with discontinuing in the index of refraction at the interface between plasma and cells make blood a multiply scattering and absorbing medium. Thus, imaging through blood is imaging of diffused and reflected by the surface structure light being scattered many times before it reaches the detector. Because of that the radiation detected at any location of back-scattered flux contains a contribution of scattering from all regions of the irradiated interior resulting in a strong background presence in the detected signals. The background masks the actual intensity signals containing imaging information about the cardiovascular surface. Along with decreasing the contrast of an image signal the scattering attenuates signal itself and limits the optical path length (OPL) in blood. One of the most difficult aspects of imaging through blood is to obtain maximal image signal contrast at maximal optical path length.
With taking into account that only the radiation collected within a field of view of the acquiring optical system is detected and that the angle of view of the cardiovascular surface, xcfx86, is different from the angle of view of the interior, xcfx86, the detected part of incident radiation I0 is described by equation
I=I0[xcfx842 xcfx81sxcfx86s+xcfx81bxcfx86]xe2x80x83xe2x80x83(1) 
where xcfx84 is the blood transmittance at the wavelength of irradiation and xcfx81s is the surface reflectance, xcfx81b is the blood diffuse reflectance.
For intraluminal irradiation, the blood can be considered as a semi-infinite medium and the term xe2x80x9cdiffuse reflectancexe2x80x9d characterizes the light emergent the semi-infinite medium back to irradiating side due to scattering. The transmittance and diffusive reflectance in eq. (1) are determined by blood optical properties characterized by an absorption coefficient, xcexca and reduced scattering coefficient (coefficient of scattering back), xcexcxe2x80x2s per unit length. In diffusion approximation of a semi-infinite cardiovascular interior the radiation backscattered by blood can be characterized by a diffuse reflection [S. Jacques, Diffuse reflectance from a semiinfinite medium. OMC and Report, May 1998] and approximated by expression:
xcfx81d∞=exp(xe2x88x92A/{square root over (3(1+N))}) 
and the equation (1) can be re-written in form:                     I        =                              I            0                    ⁡                      [                                                            ρ                  s                                ⁢                                  ϕ                  s                                ⁢                                  exp                  ⁡                                      (                                                                  -                        2                                            ⁢                                                                        3                          ⁢                                                      μ                            a                                                    ⁢                                                      μ                            t                                                                                              ⁢                      l                                        )                                                              +                              ϕ                ⁢                                  xe2x80x83                                ⁢                                  exp                  ⁡                                      (                                                                  -                        A                                                                                              3                          ⁢                                                      (                                                          1                              +                                                              N                                xe2x80x2                                                                                      )                                                                                                                )                                                                        ]                                              (        2        )            
with a total attenuation coefficient xcexct=xcexca+xcexcxe2x80x2s and diffuse reflectance characterizing parameter       N    xe2x80x2    =            μ      s      xe2x80x2              μ      a      
Here l is the optical path length, and the factor   A  =      -          ln      ⁡              (                  l                      l            0                          )            
The value of A depends on reflectance parameter Nxe2x80x2 and refractive index mismatch at the blood-air interface. As it is seen from eq. (2), the primary component of the detected intensity signals is a strong background not containing information about the vessel surface. Such strong background extremely decreases contrast of the intensity signals and dramatically shortens the viewing distance through blood. Once the radiation being diffused back by blood (the second term in eq. (2)) and reflected by the vessel surface (the first term in eq. (2)) are specified, one can calculate the contrast of the intensity signal:   K  =                    ρ        d                    ρ        s              =          exp      ⁡              [                  -                                    A              -                              3                ⁢                                  μ                  t                                ⁢                l                                                                    3                ⁢                                  μ                  t                                ⁢                                  μ                  a                                                                    ]            
Optical properties of blood are dependent on wavelength and, therefore, total attenuation and diffuse reflection are also wavelength depend. As a result the intensity signal and its contrast strongly vary with wavelength. That makes attractive to enhance the intensity signal and improve the contrast at the same time simply by selecting an xe2x80x9coptimalxe2x80x9d wavelength. International patent application WO 00/24310 published in May 2000 discloses a device and method for imaging through body fluids simply by using the radiation of mid Infrared (IR) spectrum regions (1.4 to 1.8 mkm, 2.1 to 2.4 mkm, 3.7 to 4.3 mkm, 4.6 to 5.4 mkm, and 7 to 14 mkm). Blood has obviously reduced scattering in mid IR region. As per inventors"" statement the absorption is also extremely low at mid IR wavelengths because water as the main part of blood has optical windows at these wavelengths. However, modern experimental data shows that radiation of above mentioned wavelengths is strongly absorbed by blood glucose [Jason J. Bumeister and Mark A. Arnold Spectroscopic considerations for noninvasive blood glucose measurements with near infrared spectroscopy, Infrared Spectroscopy, p. 2, 1999] and other blood components [A. Roggan et. al. Optical properties of circulating blood in the wavelength range 400-2500 nm, J. of Biomedical Optics, Vol. 4, pp. 36-46, 1999] resulting in an unacceptably short OPL. Total attenuation and diffuse reflectance spectra derived from published data are shown in FIG. 1. It is seen that total attenuation in and diffuse reflectance from blood have different spectra and at the wavelength of the signal contrast improvement (mid IR) the total attenuation is unacceptably high.
Numerous techniques for optical imaging based on special processing the detected signals are known in the art. Most of them use a transmission mode imaging. The term xe2x80x9ctransmission modexe2x80x9d is regarded to detecting the radiation passed through the turbid medium. Methods of this kind are known from the article B. Chance et. al. Highly sensitive object location in tissul models with linear m. phase and anti-phase multielement optical arrays in one and 2 dimersoons, Proc. of the National Academy of Science USA, Vol. 90 (1993) 3423-3427 and U.S. Pat. No. 5,807,262 issed Sep. 15, 1998 to Papaioannou Dimitoios. By transilluminating the turbid medium with a set of couples of beams being modulated in anti-phase the contribution of scattering to the background signals is reduced and under certain conditions the background even can be cancelled. If an object that typically absorbs the radiation is present in the medium the detected signals at the frequency of modulation becomes different from predetermined. Such technique is suitable for imaging only absorbing objects embedded in a turbid medium with wavelength independent absorbance, which obviously is not the blood. Besides that, the requirement to use a plurality of irradiating beams is not applicable for cardiovascular lumen sizes.
Various principles of reconstructing an image of a turbid medium interior by determining differences between measured and theoretically predetermined values of different parameters of passed through the turbid medium radiation are disclosed in U.S. Pat. No. 5,903,357 to Colak Sel B. issued May 11, 1999; U.S. Pat. No. 5,719,398 to Colak Sel B. issued Feb. 17, 1998 and U.S. Pat. No. 6,064,073 to Hoogenraad Johannes issued Jul. 14, 1998. Such parameters are the attenuation coefficient of a voxel of the turbid medium, intensity of light entering the turbid medium from different directions, convolution function of light transported through turbid medium. Not discussing the boundaries of applicability of used theoretical models just note that all of them consider only a large-scale homogeneous medium with spectral independent absorption. Unfortunately, the blood is a turbid medium containing large and small-scale inhomogeneities with the absorption being strongly dependent on wavelength especially in visible and near infrared spectrum [Valentin Grimblatov and A. Bekshaev, Diagnosis of lens-like biological media, Proc. SPIE, Vol. 2626, pp. 24-32, 1995] for which such technique is not applicable.
Transillumination of the turbid medium along with adjusting optical properties and measuring differences between intensity of passed through the medium radiation before and after adjustment is disclosed in U.S. Pat. No. 6,023,341 to Coiak Sel B. issued Feb. 8, 2000. Having all above described disadvantages this method of adjusting the medium optical properties by compression is principally not applicable to blood. As it has been shown in the article Valentin Grimblatov and Alexander Bekshaev, Optomechanical effect of tissue blood microcirculation under compression, Proc. SPIE, Vol. 3253, 1999 even light compression of the blood volume in cardiovascular passage brings to a non-linear changes in optical properties of blood. Besides that, in the visible spectrum that is used in this method both the total attenuation and diffuse reflection are extremely high.
A method and system of irradiating the turbid medium with anti-phase modulated dual wavelength radiation of visible spectrum (0.6 mkm-1 mkm) is disclosed in U.S. Pat. No. 5,941,827 to Papaioannon Dimitrous issued Aug. 24, 1999. Such technique can increase the image signal contrast but is suitable for viewing only absorbing objects embedded in a turbid medium with wavelength independent absorbance which obviously cannot be blood.
An alternative technique of imaging objects in turbid medium is temporal and special gating the radiation emergent from the turbid medium based on time resolved spectroscopy [U.S. Pat. No. 5,625,458 to Alfano R. R. and Polishchuk Alexander Y. issued Apr. 29, 1997, U.S. Pat. No. 5,799,656 to Alfano R. R. et al issued Sep. 1, 1998] and heterodyne processing the signals [U.S. Pat. No. 5,855,205 to Papaioannon Dimitrous and Hoolfgerf W. issued Jan. 5, 1999]. Such technique extremely increases the contrast but always shortens the viewing length.
Thus, the known techniques of optical imaging are unable for imaging cardiovascular surfaces through blood. No art has been found for technique of maximizing the viewing length along with improving the contrast of the image signal in xe2x80x9creflectance modexe2x80x9d where defecting is produced at the same side of the turbid medium as irradiating.
An object of the present invention is to provide a technique for imaging a body passage surface through the body fluids, which is free from above described disadvantages. More particularly an object of the invention is to provide a method and apparatus for imaging cardiovascular surfaces through blood capable of providing maximal viewing distance along with increased contrast of the image signals.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of imaging cardiovascular surfaces through blood, comprising the steps of intralumenally irradiating a cardiovascular interior through blood with a radiation of a first wavelength with minimum of total optical losses through blood, so that that the radiation of a first wavelength is reflected and backscattered at least partly by a cardiovascular surface, detecting all intensity signals of reflected and backscattered radiation, and processing the detected intensity signals by selecting intensity signals of radiation being backscattered by blood only, and subtracting the selected intensity signals of radiation backscattered only by blood, so as to reconstruct an image of the cardiovascular surface using the intensity signals of difference obtained by said subtracting.
In accordance with another feature of present invention, an apparatus for imaging cardiovascular surfaces through blood is provided, which includes means for intralumenally irradiating a cardiovascular interior through blood wit radiation of a first wavelength with minimum of total optical losses through blood, so that the radiation of a first wavelength is reflected and backscattered at least partly by a cardiovascular surface; means for detecting all intensity signals of reflected and backscattered radiation; and means for processing the detected intensity signals by selecting intensity signals of radiation beackscattered by blood only and subtracting the selected intensity signals of radiation backscattered only by blood from the all detected intensity signals of reflected and backscattered radiation, so as to reconstruct an image of the cardiovascular surface using the intensity signals of difference obtained by said subtracting.
For imaging through blood, a combination of spectral and spatial selection is discovered to permit maximization of the viewing length through blood and extreme increase of the signal contrast.
It will be seen that absorption and scattering of the turbid medium produce fundamentally different effect on the image signal. While absorption just attenuates radiation propagating in a turbid medium the scattering produces a dual effect. Along with contributing in signal attenuation the multiple scattering results in a strong background presence in the image signal. Accordingly, the intensity signals being used for imaging through a turbid medium, such as blood, are weak and have a dramatically low contrast. Conventional imaging technique of seeing through a turbid medium employs spectral selection of ranges in visible and mid IR and various methods of processing the intensity signals being able to increase at special conditions either the viewing length or the contrast of the intensity signals. However, none of them is applicable to see through blood and increase contrast of corresponding intensity signals at the same time.
This patent application discloses a technique of imaging the cardiovascular surface through blood by using spectral selection of radiation of a wavelength corresponding to the maximal optical path length (OPL) in blood combined with selection of radiation diffused back only by blood and differential processing of the signals. The term xe2x80x9cdifferential processingxe2x80x9d in the instant invention refers a process of producing a signal of difference between intensity signals corresponding to radiation being diffused back only by blood and by the whole vessel interior. Despite describing in this application the method and apparatus for imaging cardiovascular surfaces through blood, they are applicable for imaging the surface of any body passage through an opaque and turbid medium. Blood is considered throughout this application just as the most important body fluid.
The fundamental teaching in this patent is that scattering produces a dual effect on the intensity signals. The length of view in blood is limited by attenuation of the radiation due to absorption and scattering and a strong background presence in the intensity signal. As disclosed below there exist a spectral region in near IR where the total optical losses through blood are absolutely minimal. A principal teaching is that the line of the maximal OPL in blood classifies areas of an image zone onto those corresponding to radiation diffused back only by blood and being reflected and scattered by whole interior. The differential processing of the intensity signals of classified areas extremely increases the contrast of the signals corresponding to maximal length of view. Selection of the radiation being diffused back only by blood is based on a predetermined locating the corresponding area of the image zone.
A second principal teaching is that radiation attenuation through and diffuse reflection from blood have different optical spectra. As disclosed below a dramatically increased contrast of the image signals at maximum length of view is achieved by intraluminal irradiation of the cardiovascular interior with radiation of two wavelength one of which corresponds to maximum OPL and the other wavelength corresponds to the OPL being less than the lumen size. Having the total attenuation at the second wavelength strong enough to prevent the radiation from reaching the cardiovascular surface the background is eliminated by subtracting the intensity signals of second wavelength from those of the first wavelength.
The disclosed technique is accomplished by disclosing an apparatus utilizing above method and able for imaging cardiovascular surfaces through blood at maximum length of view with extremely improved contrast of the image signals.
A transparent to blood imaging technique has revolutionary consequences in diagnosis and treatment inside the heart or in the vasculature. Besides cardiology the disclosed imaging through blood technique would benefit several other medical disciplines, such as oncology, neurology etc.